I have experienced that SF11 glass (coated with some metal) turns transparent black/grey once metal gets removed with piranha solution.
Please state the reason and prevention.
Thanks in advance :)
Dear Aabha Bajaj , SF11 glass is a type of dense (S) Flint (F) glass, so probably it has some fraction of lead (Pb) on its composition (I couldn´t find it, sorry), and lead could be oxidized by piranha solution. On the other hand, if your SF11 glass is coated with some metal, it could be oxidized too causing the black-gray colour.
If you want to remove the metal coat aqua regia could do the job with most metals. If what you want is to clean that metal coat, then use some less aggressive cleaning solution (soap, ultrasounds...).
Be careful, if your glass item is a prism, an optical element or a fiber, it is not the same that a spare glass slide. It is always a good idea to make trials with a useless glass piece to avoid damage a valuable one.
Hope it helps.
The N-SF11 glass (SCHOTT) contains a significant amount of titania (20―30 wt%).
It seems possible that lixiviation of titanium(IV) oxide from the glass substrate can promote its later transformed precipitation over the newly roughened and hydroxylated surface, what may perhaps account for the reported darkening effect.
Such a precipitate, likely nearly amorphous, seemingly may possibly first form as sub-stoichiometric titanium(IV) oxide hydroxide, being perhaps subsequently converted to sub-stoichiometric titanium(IV) oxide hydrogensulfate and afterwards to sub-stoichiometric titanium(IV) oxide sulfate.
This hereby proposed combined lixiviation/precipitation process can perhaps be tentatively translated by the following reactions (0 < x ≤ 1):
(i) TiO2 + xH2O → TiO2-x(OH)2x;
(ii) TiO2-x(OH)2x + x(HSO4-) → TiO2-x(OH)x(HSO4)x + xOH-;
(iii) TiO2-x(OH)x(HSO4)x + x(HSO4-) → TiO2-x(HSO4)2x + xOH-;
(iv) TiO2-x(HSO4)2x + xSO42- → TiO2-x(SO4)x + 2xHSO4-.
Eventually, given suitable conditions, TiO2-x(SO4)x may even be able to further hydrolyze and form a dense network gel.
Thanks Manuel and Carlos for the insights. However it doesn't happen always. I guess while cleaning I introduced traces of normal tap water (which contains ions). Can this be a reason for the blackening of the SF11 glass?
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